Friday, August 28, 2009

ATSC Haiku

Blue screen vanishes
blue sky and green fields appear
call sign, blue screen returns

6 dB ratio
from peak to channel average
power costs a lot

no snow, no ghosting
perfect signal every time
except for drop-outs

Ricochet bankrupt
phoenix is reborn from ash
on 33 cm

MPEG2 for sound
AC-3 costs too damn much
stupid patent trolls

ATV serving all of
silicon valley

Thursday, August 27, 2009

ATSC power conundrum solved?

I've learned a bit more about some of the features of the HP 8590L spectrum analyzer I got off eBay a while ago. One feature it has is the ability to calculate channel average power and the adjacent channel power ratio (that is, how many dB down the power is on the 6 MHz above and below the channel of interest). And at the preferred settings of the exciter, the output power is about 15 watts, with the adjacent channels being down about 35 dB. That's a little more out-of-channel garbage than I had hoped for - a bit more than 20 mW of ERP. But even so, on 900 MHz that small amount of power should die easily within half a mile of the site. Heck, I've transmitted with the bare exciter at approximately that power level and been unable to receive it half a block away from home. So I'm not going to worry about it. As always, reducing power will reduce the reconstituted sidebands, though the bare exciter has sidebands that are only about 48 dB down from the main signal, so there's only so much more improvement to be had.

Another bit of good news is that I spent some time "boiling the dummy load" this evening. Without the muffin fan, the heat sink of the amp gets so hot that you can't comfortably touch it. But with the muffin fan sitting on top and running, the heat sink stays almost as cool as the ambient air temperature (almost). So that bodes really well for the health of the amp when it's put in service.

Monday, August 24, 2009

First words from the moon?

If you ask people what were the first words spoken by a human from the moon's surface, their answer will often be Neil Armstrong's "One small step" quotation. I happened to think about that a bit today, and it doesn't really ring true.

As of yet, no human being has truly been in contact with the lunar surface. The only way such an event would truly be possible would be to build a pressurized enclosure on the moon. Only then could someone walk on the surface without the protection of a space suit. What we're left with is an approximation - the Apollo astronauts who visited the lunar surface did so constantly surrounded by a bubble of Earth's atmosphere, whether that bubble was located around them in a suit while they were EVA or whether that bubble was around them in the LEM. And Neil Armstrong and Buzz Aldrin spent an extended period of time after the actual landing before they opened the hatch and descended to the actual lunar surface. Buzz even had rime to take communion.

So how do you pick the first words spoken from the moon? Well, they'd have to be the first words spoken by a human being who was inside of a vessel that was in contact with the lunar surface. And if you read the NASA transcripts of Apollo 11, you'll find that the first words from the lunar surface given that definition actually came from Buzz Aldrin.

As the LEM was descending for its final landing, a probe descending from one of the landing legs made contact with the lunar surface. That probe lit a light on the instrument panel. When that light lit up, Buzz said, "Contact light," as a cue to Neil to cut the engines and let the LEM fall the rest of the way to the surface.

And those words were the ones that made history.

History is often like that. The first creatures ever to cross the Golden Gate other than by water or air were a pair of workmen on the Golden Gate bridge who were engaging in a repair mission of sorts. They hauled themselves over the unfinished cables and into the history books more than a year before the first pedestrian would cross the finished bridge. The flag raising at Mount Suribachi (on Iwo Jima) that was widely and famously photographed was actually the raising of the second flag, ordered by a commander who thought the first flag was too small. If you visit the Amundsen-Scott South Pole station, you'll find two poles - the real South Pole, which is just a simple spike in the ground and a small, plain sign, and a few hundred yards away, is the so-called "ceremonial" pole, which is a giant barber-pole decorated monstrosity surrounded by the flags of all the nations with a presence in Antarctica. The ceremonial one is the one that people expect to see, but it's not the real one.

Final TX configuration set

I've set up the transmitter in what I believe will be its final configuration. I put a 6 dB attenuator on the output of the exciter to raise the output power setting required so that it would be in the middle of its range rather than on the end. Getting the exciter output to an average power of about 6 dBm now means setting the output power to 10 instead of 3. I traded some mail with the guys at SR-Systems and they said that the "sweet spot" is anywhere between 5 and 13. Anyway, with 6 dBm of power, the amp is supposed to generate 15 watts of power, which means an output ERP of about 75 watts. I put an N to UHF adapter in the input of the spectrum analyzer and then put a big nail in the center conductor socket and laid the coax next to it that was hooked up between the amp and the dummy load. It may be less than perfectly accurate, but my hope is that it's close enough to score. Anyway, with that setup, I could see the "ramps" of out-of-channel noise well enough to check how strong they were relative to the ATSC pilot (which, again, is 11.3 dB down from the channel average power). At a power setting of 10, the skirts were just about 47 dB down from the average power, which is right where the part 73 channel mask would have them. At a power level of 11, they come up to closer to 40 dB down. Now, the part 73 channel mask is not of any particular relevance for part 97 operations, but we amateurs are required to reduce spurious emissions in accordance with good engineering practice. Besides, limiting the output power can't help but make the amplifier just that much happier, which means it is likely to last longer - particularly given that it will be in a repeater shack that is not climate controlled.

I have a 20 watt 20 dB attenuator arriving in the mail tomorrow, so I'll be able to run the output of the amp directly into the SA for a final check tomorrow evening. Not only will I be able to directly measure the out-of-channel emissions, but I'll be able to get an accurate power output reading as well.

So it will put out 75 watts of ERP (perhaps it might drop down to 70 or so with coax losses when all is said and done). That still should be receivable everywhere the analog signal of K6BEN is receivable today.

Get your popcorn ready

So, the 33 cm ATSC transmitter will hopefully be going into more or less full time service sometime soon now.

So just to get people prepared, let me outline what you'll need in order to see the transmissions.

First and foremost, you must have a line-of-sight to the east San Jose foothills. And it has to be a real line of sight. That means that on a clear day you can look in the direction of Mount Hamilton and see the mountains. A single tree in the way will knock your received signal strength down by 20 dB - 100 times!

At the location where you have that nice, clear line of sight towards Mount Hamilton, you'll need to mount a 900 MHz directional antenna. I'm about 10 miles away from where the transmitter is going to be located, and I can receive it with a 7 element 10 dBi 900 MHz Yagi. I bought it off the Internet for about $30. If you're further away, or if you have trees in the way, you'll need something bigger, like the Directive Systems 3318LYARM 18 element 16.5 dBi loop Yagi.

You'll need to use as short a run of coax as possible from the antenna to the downconverter, and the best coax you can afford. Losses at 900 MHz can be severe. I use LMR-400.

The best downconverter I've found can be had from P.C. Electronics. You'll want the TVC-9S 900 MHz PLL controlled downconverter. From there, you can run ordinary 75 ohm cable TV coax from the downconverter to your ATSC tuner or TV. Set the downconverter to channel 2, which maps 910-916 MHz to TV channel 3.

As a bonus, if you have a good clear line of sight to Mt. Diablo and an analog TV, you can try picking up the W6CX analog ATV repeater output on 918-924 MHz.

You'll need to perform a channel scan while the transmitter is active in order to discover the downconverted signal on channel 3. Fortunately, the K6BEN repeater is active from 0 to 10 minutes past each hour transmitting ID slides. So your best bet is to scan at about 2 minutes past the hour. Of course, if your tuner/TV is capable of tuning directly to a channel without scanning, you can simply tune it to channel 3.

Saturday, August 22, 2009

New Amateur ATSC DX record - complete with video

I went up to the Sierra Rd QTH in the east foothills, and this time everything went perfectly. I was able to capture a video from up there, and with a good line of sight and 80 watts of ERP, I was able to get a signal strength of 100 and a signal quality of 75 or so on the HD HomeRun. This location is very similar to the location of the K6BEN repeater and is in, more or less, the same direction. Google Earth says it's 9.07 miles away from home. The coordinates are 37.413081° N, 121.824316° W.

Here's a screenshot from my phone's ssh client showing the status report from the HDHomeRun with the signal strength and qualty (the first line showing no reception was a control - the transmitter was unkeyed):

It was actually sort of tough to get the phone to cooperate. Cell phones operate in the 800 MHz range, and transmitting 85 watts of 900 MHz right next to the phone desensitized the receiver somewhat. I did manage to get working in enough fits and starts to check the signal strength and start and stop the recording, thank goodness.

The receive antenna was a 7 element (10 dBi) Yagi - about $30 off the Internet, and the reception was almost perfect. That pretty much proves that this transmitter is going to work. With a real antenna - like the 18 dBi loop Yagi I have on order - reception should be possible from perhaps 30 miles away with a good line of sight.

Unlike the first time, it was not necessary to adjust the power. The exciter was set to power level "3", which is an average power of about 0 dBm, or a PEP of about 6 dBm. That means an output power of about 16 watts or so, for an ERP of 80 watts. Of course, all of that is in theory. Still on my to-do list is to run this setup through a proper sampling wattmeter and dummy load to get a measured output power level and get a good measurement on how far down the out-of-channel emissions are. Part 73 contains an emissions mask requirement that says that the out-of-channel emissions must be reduced from the channel average power by at least 47 dB - that is, 36 dB below the pilot. That's a pretty reasonable goal to shoot for. At the power we're contemplating, it'd be less than a couple milliwatts. At 900 MHz, that ought not to be a problem.

Tuesday, August 18, 2009

A major amateur ATSC milestone achieved

I just got back from a "DXpedition" of sorts. I went to the crest of a hill in Cupertino and transmitted ATSC at 910-916 MHz and successfully received a picture at home.

The path length, according to Google Earth, is 6.59 miles. I now claim that as the amateur ATSC DX record. I'll be happy to be proven wrong, but so far as I know, nobody apart from a professional broadcaster has transmitted an ATSC signal and successfully received it at that great a distance.

Alas, I don't have any recordings. In order to make this work, I connected the TVC-9S to the best tuner in the house - our brand new Samsung 52" LCD TV in the living room. I then drove off into the night and called Scarlet on the cell phone and asked her to watch. The reception wasn't perfect, but it was there for reasonable stretches.

The receive antenna was a 7 element 900 MHz Yagi I bought off the Internet for about $30. I'm still waiting for the 15 element loop Yagi from Directive Systems - that should give me an 8 dB boost in receive gain when it comes. So the total receive gain was on the order of 7.8 dBd, taking a small loss for coax.

On the transmit power side, that's a much grayer area.

Recall the following rules of thumb: If you can see the ATSC pilot on the spectrum analyzer, it's power level is approximately 11 dB lower than the channel average power level. The peak-to-average power ratio for ATSC is about 6 dB. So the pilot to PEP ratio is 17 dB.

I started out with the minimod power set to "3" on a scale of 1 to 15. On the spectrum analyzer, that sets the pilot at about -11 dBm, which means the channel average power is about 0 dBm. Add 6 dB for the peak-to-average ratio, and that's an input PEP of 6 dBm. The power curve of the 3370PAHS has that being an output power of about 47 dBm (50 watts), or an average power of about 12 watts. That's right between the 1 and 2 dB compression points for the amp. That signal didn't quite make it. I had actually made two trips out to the transmit location - the first time was with the HDHomeRun set up as the receiver, and that configuration did generate a signal strength of about 70, but a signal quality of 0. With the TV hooked up, I didn't have ready access to any signal metrics, so it was just whether it would decode at all or not.

In a fit of impatience, however, I decided to crank the power up, just to see. And sure enough, a power setting of "12" was what it took for decent reception.

Now, that's a pilot level of about 0 dBm, or an average input power of 11 dBm, or an input PEP of 17 dBm. The amp's specifications show it is fully saturated at 16 dBm input power, for an output power of 100 watts PEP.

I can't for the life of me imagine that the amp was very happy doing that.

I can't look at the output of the amp on the spectrum analyzer, since it exceeds the maximum input level of 1 watt. I do have a 25 watt 20 dB attenuator on order, however. When it gets here I'll be able to look and figure out exactly how terrible the splatter is from running the exciter that hot. If previous experience on 420 MHz is any guide, it won't be pretty.

What I can't quite wrap my head around, though, is that if the amp is being overdriven, how can it be that that actually improves reception? Surely an amp that's heavily clipping ATSC would make the eyes close up like a punch drunk boxer's.

We'll have to wait and see. In the meantime, I'm happy to claim some small success, and I'd like to publicly thank my lovely wife Scarlet for helping me verify this transmission. I couldn't have done it - or anything thus far - without her unending help, assistance and understanding.

Friday, August 14, 2009

YouTube DIY TV antennas

I was fooling around today and ran across a collection of videos on YouTube talking about making homemade "HD" TV antennas. Just about all of them are 4 bay bowtie dipole arrays. Pretty much a Channel Master 4221 without the screen, but usually built by screwing coat-hanger wire to a 2x4.

That'll work ok, but there are a few caveats that I feel duty bound to point out:

1. Measure carefully. All the measurements are fairly critical on an antenna like this. Particularly the length of the elements and the distance between them.

2. The CM4221 has a reflector screen behind it. This gives the antenna F/B ratio that it otherwise wouldn't have. Particularly around here, this is absolutely necessary to reduce multipath. The 4221HD has a modified screen that acts like a VHF-hi dipole. This allows it to receive channels 7-13 in addition to 14-51.

3. Put two of these next to each other (again, measuring the distance carefully) and connect them together and you'll have an 8 bay - the equivalent to the CM 4228, one of the best UHF antennas out there.

4. If you put this antenna outdoors, you're going to either need to build it far more robustly than most of the videos show, or you'll need to replace it every few years as it corrodes.

5. If you put this antenna up indoors, you're fooling yourself. For such an antenna to exhibit its designed gain, it must be mounted several wavelengths away from other objects. Because of the huge amount of multipath involved, indoor reception is a crapshoot. It always has been. It's just that with analog, it was easier to make do with a worse signal, or to turn a bad one into a good one experimentally (what I like to call "Antenna Twister").

6. Some of these video describe this as an "HD" antenna. Even Channel Master is somewhat guilty of that in adding "HD" to the model numbers of some of their antennas. No. Antennas do not care about the nature of the modulation of the signals they receive. Antennas care about frequency. And TV post-transition is using the same frequencies as it was before (fewer of them, actually, since the top 100 MHz of UHF has been reallocated). So a TV antenna is a TV antenna and always has been. And TV antennas have not significantly changed since the TV bands were fixed after World War II.

Monday, August 10, 2009

Mineta International: Best at what?

San Jose's airport was recently named the airport of the year. Yes, you heard that right. If you've ever had the misfortune to have to use that airport, you may be confused as to how that never ending construction zone, where the roads are never configured the same way twice in a row, could possibly win anything that isn't a boobie prize. If you read the article, the reason becomes obvious: they didn't ask the passengers. They asked the pilots. It's a little like asking the bus drivers' union to name the route of the year.

Mineta International is a fitting tribute to our former congressman: provincial, backwards, and perpetually being reconstructed. Quite honestly, the best reason to stay alive is to see which transportation boondoggle gets finished first: BART to San Jose or the Airport.

Saturday, August 8, 2009

Test result

Since I didn't give much advance notice for this test, I wasn't expecting much response. Really, what I was going to do was to see if I could receive anything with the HDHomeRun being driven by the TV antenna feeding into a TVC-9S downconverter.

Well, from Cañada College, I was unable to even tickle the signal strength meter in the HDHomeRun - it couldn't tell the difference between the transmitter being on or off.

But I didn't give up. I went by a spot I know about in the Cupertino foothills - near Miles' old house (for those of you who know Miles). From there, I set the transmitter up again and this time I was able to bump up the received signal strength by 10 points, and make the signal quality gauge go up from 0 to about 5 at one point.

That's a path length of a little over 6 miles (compared to almost 19 miles for Cañada). A LR analysis of the path between Cañada College and my house compared to the Cupertino hills QTH and my house shows a path loss difference of 12 dB or so. The predicted field strength for the Cupertino hills QTH is 76.7 dBµV/m and for Cañada is 65.21 dBµV/m.

Keep in mind that's with a receive antenna pointed the wrong way by about 45 degrees and tuned to a band almost double the wavelength. In other words, I'm relatively sure I was paying quite a receive penalty with that setup. But I still managed to tickle it.

I have a proper 33 cm loop Yagi antenna on order, but it hasn't arrived yet. When it does, I'm confident that I'll be able to pull in the signal from the Cupertino QTH at least.

Test at 1

I've got to head out for ham tests, but after that I will be at Cañada College at 1 as promised transmitting ATSC at 910-916 MHz with about 150 watts of ERP. If you have a TV antenna and a spectrum analyzer, and you're in the South Bay or Southern Peninsula (South of San Carlos), please hook the two together and see if you can see anything. Thanks in advance, and fingers crossed!

I will be on Charlie (WA6TEM/R: 147.855-, PL 100Hz) monitoring for reports or questions or what not.

Hope someone can see me!

Friday, August 7, 2009

Hall of Vaporware

So, where the hell are these products we were promised?

  • TomTom for iPhone
  • iPhone tethering
  • iPhone multimedia SMS
  • AT&T Femtocell
  • Next generation DirecTiVo
  • And, of course, no such list would be complete without mentioning the Flying Cars

Saturday, August 1, 2009

68 channels and counting!

KAXT-CA lit up their digital transmitter today (or sometime in the last couple days). They're RF channel is 42, taking over the old KTNC analog spot. They join the quad of stations on Mt. Allison. Their analog transmitter over in the East San Jose foothills is still up on channel 22. It's unclear how long it will stay.

But get this: They're transmitting twelve streams! Most of them are foreign language or religious or shopping, so there probably isn't a lot I'll be watching. And, of course, to fit 12 streams in 19 MB/s, they've had to squeeze the hell out of them. But their signal is good and strong.

So welcome to the digital age, KAXT!

There are only a couple stations left to show up. One is KMMC. They're set to flash-cut to digital on channel 40 sometime next month. Another is KTVJ, which will begin transmitting from Mt. Tamalpais on channel 4 sometime next month. Although there is a clear line of sight from here to Mt. Tamalpais, it'll be interesting to see how challenging it is to receive VHF low ATSC from so far away. The electrical noise might wind up drowning them out. Not to mention I'll probably need to build a dipole or perhaps a 2 or 3 element beam to pull them in (did I mention that a dipole for 70 MHz is gonna be almost 10 feet long?).